Smoke
opacity is a measurement of light extinction, or
the blackness of the exhaust plume, expressed as
a percentage. 100% opacity would be completely dark,
and 0% would be totally transparent. The black smoke
we see in diesel exhaust is composed primarily of
carbon particles resulting from the combustion of
diesel fuel.

The
opacity limit is measured at the peak, or darkest
point, of the exhaust cloud. This measurement is
most commonly taken during the snap acceleration
test, where the vehicle is at idle, with all brakes
off, and the throttle is "punched" to
the floorboard, initiating the exhaust cloud. Opacity
is measured as a surrogate for particulate matter.

The
table below illustrates the approximate opacity
of our current emission
standards.

HDDVs
are subject to opacity standards based on the production
year of the vehicles' engine. The standards were
devised to take into account the state of engine
technology, and typical wear conditions. The
opacity standards are: